1================================================================
2Documentation for Kdump - The kexec-based Crash Dumping Solution
3================================================================
4 5This document includes overview, setup and installation, and analysis
6information.
7 8Overview
9========
10 11Kdump uses kexec to quickly boot to a dump-capture kernel whenever a
12dump of the system kernel's memory needs to be taken (for example, when
13the system panics). The system kernel's memory image is preserved across
14the reboot and is accessible to the dump-capture kernel.
15 16You can use common commands, such as cp and scp, to copy the
17memory image to a dump file on the local disk, or across the network to
18a remote system.
19 20Kdump and kexec are currently supported on the x86, x86_64, ppc64, ia64,
21and s390x architectures.
22 23When the system kernel boots, it reserves a small section of memory for
24the dump-capture kernel. This ensures that ongoing Direct Memory Access
25(DMA) from the system kernel does not corrupt the dump-capture kernel.
26The kexec -p command loads the dump-capture kernel into this reserved
27memory.
28 29On x86 machines, the first 640 KB of physical memory is needed to boot,
30regardless of where the kernel loads. Therefore, kexec backs up this
31region just before rebooting into the dump-capture kernel.
32 33Similarly on PPC64 machines first 32KB of physical memory is needed for
34booting regardless of where the kernel is loaded and to support 64K page
35size kexec backs up the first 64KB memory.
36 37For s390x, when kdump is triggered, the crashkernel region is exchanged
38with the region [0, crashkernel region size] and then the kdump kernel
39runs in [0, crashkernel region size]. Therefore no relocatable kernel is
40needed for s390x.
41 42All of the necessary information about the system kernel's core image is
43encoded in the ELF format, and stored in a reserved area of memory
44before a crash. The physical address of the start of the ELF header is
45passed to the dump-capture kernel through the elfcorehdr= boot
46parameter. Optionally the size of the ELF header can also be passed
47when using the elfcorehdr=[size[KMG]@]offset[KMG] syntax.
48 49 50With the dump-capture kernel, you can access the memory image, or "old
51memory," in two ways:
52 53- Through a /dev/oldmem device interface. A capture utility can read the
54 device file and write out the memory in raw format. This is a raw dump
55 of memory. Analysis and capture tools must be intelligent enough to
56 determine where to look for the right information.
57 58- Through /proc/vmcore. This exports the dump as an ELF-format file that
59 you can write out using file copy commands such as cp or scp. Further,
60 you can use analysis tools such as the GNU Debugger (GDB) and the Crash
61 tool to debug the dump file. This method ensures that the dump pages are
62 correctly ordered.
63 64 65Setup and Installation
66======================
67 68Install kexec-tools
69-------------------
70 711) Login as the root user.
72 732) Download the kexec-tools user-space package from the following URL:
74 75http://kernel.org/pub/linux/utils/kernel/kexec/kexec-tools.tar.gz 76 77This is a symlink to the latest version.
78 79The latest kexec-tools git tree is available at:
80 81git://git.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git
82and
83http://www.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git 84 85There is also a gitweb interface available at
86http://www.kernel.org/git/?p=utils/kernel/kexec/kexec-tools.git 87 88More information about kexec-tools can be found at
89http://horms.net/projects/kexec/ 90 913) Unpack the tarball with the tar command, as follows:
92 93 tar xvpzf kexec-tools.tar.gz
94 954) Change to the kexec-tools directory, as follows:
96 97 cd kexec-tools-VERSION
98 995) Configure the package, as follows:
100 101 ./configure
102 1036) Compile the package, as follows:
104 105 make
106 1077) Install the package, as follows:
108 109 make install
110 111 112Build the system and dump-capture kernels
113-----------------------------------------
114There are two possible methods of using Kdump.
115 1161) Build a separate custom dump-capture kernel for capturing the
117 kernel core dump.
118 1192) Or use the system kernel binary itself as dump-capture kernel and there is
120 no need to build a separate dump-capture kernel. This is possible
121 only with the architectures which support a relocatable kernel. As
122 of today, i386, x86_64, ppc64 and ia64 architectures support relocatable
123 kernel.
124 125Building a relocatable kernel is advantageous from the point of view that
126one does not have to build a second kernel for capturing the dump. But
127at the same time one might want to build a custom dump capture kernel
128suitable to his needs.
129 130Following are the configuration setting required for system and
131dump-capture kernels for enabling kdump support.
132 133System kernel config options
134----------------------------
135 1361) Enable "kexec system call" in "Processor type and features."
137 138 CONFIG_KEXEC=y
139 1402) Enable "sysfs file system support" in "Filesystem" -> "Pseudo
141 filesystems." This is usually enabled by default.
142 143 CONFIG_SYSFS=y
144 145 Note that "sysfs file system support" might not appear in the "Pseudo
146 filesystems" menu if "Configure standard kernel features (for small
147 systems)" is not enabled in "General Setup." In this case, check the
148 .config file itself to ensure that sysfs is turned on, as follows:
149 150 grep 'CONFIG_SYSFS' .config
151 1523) Enable "Compile the kernel with debug info" in "Kernel hacking."
153 154 CONFIG_DEBUG_INFO=Y
155 156 This causes the kernel to be built with debug symbols. The dump
157 analysis tools require a vmlinux with debug symbols in order to read
158 and analyze a dump file.
159 160Dump-capture kernel config options (Arch Independent)
161-----------------------------------------------------
162 1631) Enable "kernel crash dumps" support under "Processor type and
164 features":
165 166 CONFIG_CRASH_DUMP=y
167 1682) Enable "/proc/vmcore support" under "Filesystems" -> "Pseudo filesystems".
169 170 CONFIG_PROC_VMCORE=y
171 (CONFIG_PROC_VMCORE is set by default when CONFIG_CRASH_DUMP is selected.)
172 173Dump-capture kernel config options (Arch Dependent, i386 and x86_64)
174--------------------------------------------------------------------
175 1761) On i386, enable high memory support under "Processor type and
177 features":
178 179 CONFIG_HIGHMEM64G=y
180 or
181 CONFIG_HIGHMEM4G
182 1832) On i386 and x86_64, disable symmetric multi-processing support
184 under "Processor type and features":
185 186 CONFIG_SMP=n
187 188 (If CONFIG_SMP=y, then specify maxcpus=1 on the kernel command line
189 when loading the dump-capture kernel, see section "Load the Dump-capture
190 Kernel".)
191 1923) If one wants to build and use a relocatable kernel,
193 Enable "Build a relocatable kernel" support under "Processor type and
194 features"
195 196 CONFIG_RELOCATABLE=y
197 1984) Use a suitable value for "Physical address where the kernel is
199 loaded" (under "Processor type and features"). This only appears when
200 "kernel crash dumps" is enabled. A suitable value depends upon
201 whether kernel is relocatable or not.
202 203 If you are using a relocatable kernel use CONFIG_PHYSICAL_START=0x100000
204 This will compile the kernel for physical address 1MB, but given the fact
205 kernel is relocatable, it can be run from any physical address hence
206 kexec boot loader will load it in memory region reserved for dump-capture
207 kernel.
208 209 Otherwise it should be the start of memory region reserved for
210 second kernel using boot parameter "crashkernel=Y@X". Here X is
211 start of memory region reserved for dump-capture kernel.
212 Generally X is 16MB (0x1000000). So you can set
213 CONFIG_PHYSICAL_START=0x1000000
214 2155) Make and install the kernel and its modules. DO NOT add this kernel
216 to the boot loader configuration files.
217 218Dump-capture kernel config options (Arch Dependent, ppc64)
219----------------------------------------------------------
220 2211) Enable "Build a kdump crash kernel" support under "Kernel" options:
222 223 CONFIG_CRASH_DUMP=y
224 2252) Enable "Build a relocatable kernel" support
226 227 CONFIG_RELOCATABLE=y
228 229 Make and install the kernel and its modules.
230 231Dump-capture kernel config options (Arch Dependent, ia64)
232----------------------------------------------------------
233 234- No specific options are required to create a dump-capture kernel
235 for ia64, other than those specified in the arch independent section
236 above. This means that it is possible to use the system kernel
237 as a dump-capture kernel if desired.
238 239 The crashkernel region can be automatically placed by the system
240 kernel at run time. This is done by specifying the base address as 0,
241 or omitting it all together.
242 243 crashkernel=256M@0
244 or
245 crashkernel=256M
246 247 If the start address is specified, note that the start address of the
248 kernel will be aligned to 64Mb, so if the start address is not then
249 any space below the alignment point will be wasted.
250 251 252Extended crashkernel syntax
253===========================
254 255While the "crashkernel=size[@offset]" syntax is sufficient for most
256configurations, sometimes it's handy to have the reserved memory dependent
257on the value of System RAM -- that's mostly for distributors that pre-setup
258the kernel command line to avoid a unbootable system after some memory has
259been removed from the machine.
260 261The syntax is:
262 263 crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset]
264 range=start-[end]
265 266 'start' is inclusive and 'end' is exclusive.
267 268For example:
269 270 crashkernel=512M-2G:64M,2G-:128M
271 272This would mean:
273 274 1) if the RAM is smaller than 512M, then don't reserve anything
275 (this is the "rescue" case)
276 2) if the RAM size is between 512M and 2G (exclusive), then reserve 64M
277 3) if the RAM size is larger than 2G, then reserve 128M
278 279 280 281Boot into System Kernel
282=======================
283 2841) Update the boot loader (such as grub, yaboot, or lilo) configuration
285 files as necessary.
286 2872) Boot the system kernel with the boot parameter "crashkernel=Y@X",
288 where Y specifies how much memory to reserve for the dump-capture kernel
289 and X specifies the beginning of this reserved memory. For example,
290 "crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory
291 starting at physical address 0x01000000 (16MB) for the dump-capture kernel.
292 293 On x86 and x86_64, use "crashkernel=64M@16M".
294 295 On ppc64, use "crashkernel=128M@32M".
296 297 On ia64, 256M@256M is a generous value that typically works.
298 The region may be automatically placed on ia64, see the
299 dump-capture kernel config option notes above.
300 301 On s390x, typically use "crashkernel=xxM". The value of xx is dependent
302 on the memory consumption of the kdump system. In general this is not
303 dependent on the memory size of the production system.
304 305Load the Dump-capture Kernel
306============================
307 308After booting to the system kernel, dump-capture kernel needs to be
309loaded.
310 311Based on the architecture and type of image (relocatable or not), one
312can choose to load the uncompressed vmlinux or compressed bzImage/vmlinuz
313of dump-capture kernel. Following is the summary.
314 315For i386 and x86_64:
316 - Use vmlinux if kernel is not relocatable.
317 - Use bzImage/vmlinuz if kernel is relocatable.
318For ppc64:
319 - Use vmlinux
320For ia64:
321 - Use vmlinux or vmlinuz.gz
322For s390x:
323 - Use image or bzImage
324 325 326If you are using a uncompressed vmlinux image then use following command
327to load dump-capture kernel.
328 329 kexec -p <dump-capture-kernel-vmlinux-image> \
330 --initrd=<initrd-for-dump-capture-kernel> --args-linux \
331 --append="root=<root-dev> <arch-specific-options>"
332 333If you are using a compressed bzImage/vmlinuz, then use following command
334to load dump-capture kernel.
335 336 kexec -p <dump-capture-kernel-bzImage> \
337 --initrd=<initrd-for-dump-capture-kernel> \
338 --append="root=<root-dev> <arch-specific-options>"
339 340Please note, that --args-linux does not need to be specified for ia64.
341It is planned to make this a no-op on that architecture, but for now
342it should be omitted
343 344Following are the arch specific command line options to be used while
345loading dump-capture kernel.
346 347For i386, x86_64 and ia64:
348 "1 irqpoll maxcpus=1 reset_devices"
349 350For ppc64:
351 "1 maxcpus=1 noirqdistrib reset_devices"
352 353For s390x:
354 "1 maxcpus=1 cgroup_disable=memory"
355 356Notes on loading the dump-capture kernel:
357 358* By default, the ELF headers are stored in ELF64 format to support
359 systems with more than 4GB memory. On i386, kexec automatically checks if
360 the physical RAM size exceeds the 4 GB limit and if not, uses ELF32.
361 So, on non-PAE systems, ELF32 is always used.
362 363 The --elf32-core-headers option can be used to force the generation of ELF32
364 headers. This is necessary because GDB currently cannot open vmcore files
365 with ELF64 headers on 32-bit systems.
366 367* The "irqpoll" boot parameter reduces driver initialization failures
368 due to shared interrupts in the dump-capture kernel.
369 370* You must specify <root-dev> in the format corresponding to the root
371 device name in the output of mount command.
372 373* Boot parameter "1" boots the dump-capture kernel into single-user
374 mode without networking. If you want networking, use "3".
375 376* We generally don' have to bring up a SMP kernel just to capture the
377 dump. Hence generally it is useful either to build a UP dump-capture
378 kernel or specify maxcpus=1 option while loading dump-capture kernel.
379 380* For s390x there are two kdump modes: If a ELF header is specified with
381 the elfcorehdr= kernel parameter, it is used by the kdump kernel as it
382 is done on all other architectures. If no elfcorehdr= kernel parameter is
383 specified, the s390x kdump kernel dynamically creates the header. The
384 second mode has the advantage that for CPU and memory hotplug, kdump has
385 not to be reloaded with kexec_load().
386 387* For s390x systems with many attached devices the "cio_ignore" kernel
388 parameter should be used for the kdump kernel in order to prevent allocation
389 of kernel memory for devices that are not relevant for kdump. The same
390 applies to systems that use SCSI/FCP devices. In that case the
391 "allow_lun_scan" zfcp module parameter should be set to zero before
392 setting FCP devices online.
393 394Kernel Panic
395============
396 397After successfully loading the dump-capture kernel as previously
398described, the system will reboot into the dump-capture kernel if a
399system crash is triggered. Trigger points are located in panic(),
400die(), die_nmi() and in the sysrq handler (ALT-SysRq-c).
401 402The following conditions will execute a crash trigger point:
403 404If a hard lockup is detected and "NMI watchdog" is configured, the system
405will boot into the dump-capture kernel ( die_nmi() ).
406 407If die() is called, and it happens to be a thread with pid 0 or 1, or die()
408is called inside interrupt context or die() is called and panic_on_oops is set,
409the system will boot into the dump-capture kernel.
410 411On powerpc systems when a soft-reset is generated, die() is called by all cpus
412and the system will boot into the dump-capture kernel.
413 414For testing purposes, you can trigger a crash by using "ALT-SysRq-c",
415"echo c > /proc/sysrq-trigger" or write a module to force the panic.
416 417Write Out the Dump File
418=======================
419 420After the dump-capture kernel is booted, write out the dump file with
421the following command:
422 423 cp /proc/vmcore <dump-file>
424 425You can also access dumped memory as a /dev/oldmem device for a linear
426and raw view. To create the device, use the following command:
427 428 mknod /dev/oldmem c 1 12
429 430Use the dd command with suitable options for count, bs, and skip to
431access specific portions of the dump.
432 433To see the entire memory, use the following command:
434 435 dd if=/dev/oldmem of=oldmem.001
436 437 438Analysis
439========
440 441Before analyzing the dump image, you should reboot into a stable kernel.
442 443You can do limited analysis using GDB on the dump file copied out of
444/proc/vmcore. Use the debug vmlinux built with -g and run the following
445command:
446 447 gdb vmlinux <dump-file>
448 449Stack trace for the task on processor 0, register display, and memory
450display work fine.
451 452Note: GDB cannot analyze core files generated in ELF64 format for x86.
453On systems with a maximum of 4GB of memory, you can generate
454ELF32-format headers using the --elf32-core-headers kernel option on the
455dump kernel.
456 457You can also use the Crash utility to analyze dump files in Kdump
458format. Crash is available on Dave Anderson's site at the following URL:
459 460http://people.redhat.com/~anderson/ 461 462 463To Do
464=====
465 4661) Provide relocatable kernels for all architectures to help in maintaining
467 multiple kernels for crash_dump, and the same kernel as the system kernel
468 can be used to capture the dump.
469 470 471Contact
472=======
473 474Vivek Goyal (vgoyal@redhat.com)
475Maneesh Soni (maneesh@in.ibm.com)
476 477